The 7 cycling mistakes that crash new aquariums. How to do a fishless cycle correctly + when to add fish.
The #1 cause of "my fish died after a week" is ammonia poisoning from an uncycled tank. Cycle takes 2-6 weeks. ALWAYS verify with a test kit that ammonia + nitrite both convert to 0 within 24 hours of dosing 2 ppm ammonia before adding fish.
Household ammonia often contains surfactants, fragrances, and detergents that kill bacteria. Use ONLY pure ammonium hydroxide or chloride. Brands: Dr. Tim's Ammonium Chloride, Ace Hardware Janitorial Ammonia (verify label is pure).
New filter floss is fine but biological media (ceramic rings, bio-balls, sponges) holds the bacterial colony. Replace mechanical only; never replace biological media all at once.
Chlorine + chloramine kill the cycling bacteria. ALWAYS use Prime/dechlorinator OR RODI water for water changes during and after cycling.
"It's probably done" is the most expensive guess in the hobby. Test ammonia + nitrite + nitrate every 3 days during cycling. Only add fish when both ammonia + nitrite read 0 within 24 hours of a 2 ppm ammonia dose.
Fishless cycle = fishless. If you want to do a fish-in cycle, ONLY use a few hardy guppies/danios + dose Prime daily + small water changes weekly. Most fish die during fish-in cycles.
After cycling, nitrate accumulates to 30-80 ppm. Do a 50% water change BEFORE adding livestock to drop nitrate to safe levels (under 20 ppm).
Browse the full common mistakes library.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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